Lateral wellbores, or "laterals", offer the potential to drain more oil than would be recovered otherwise. For example, laterals may be used to tap fresh oil by intersecting fractures, penetrating pay discontinuities, and draining up-dip traps. Lateral recompletions can also correct production problems such as water coning, gas coning, and excessive water cuts from hydraulic fractures which extend below the oil-water interface. Moreover, synergistic benefits may result from coupling lateral recompletions with enhanced recovery techniques to solve conformance problems, to contact unswept oil by recompleting injection wells, and to redirect sweep by converting existing well patterns into line-drive configurations. Finally lateral recompletion strategies can take advantage of current production infrastructure, capital resources of existing wellbores, known resources of oil in place, and secondary and tertiary recovery technology.
One major impediment to the widespread use of lateral re-entries is that the cost of drilling and completing laterals should be kept as low as possible. Workover economics in mature fields require substantial cost reductions over the methods most often used for drilling new horizontal wells. Thus, there is a great need for a reliable reduced-cost drilling system that utilizes the equipment and cost structures of workover and repair services.
In addition, to the economic constraints, there are technical limitations. For a curve drilling system to be technically successful it should preferably drill a consistent radius of curvature and drill the curve in the desired direction. This is because it is highly desireable to:
Position the end of the drilling assembly within a precise depth interval so the lateral can traverse the pay zone as desired. PA1 Place the lateral in a direction dictated by well spacing, desired sweep pattern, or other geological considerations. PA1 Establish a smooth wellbore to facilitate drilling the lateral and completing the well.
Rotary-steerable drilling systems are one category of curve drilling systems. The downhole components of such systems often include a curve assembly, flexible drill collars, and orientation equipment. The curve assembly is relatively short and incorporates a flexible joint that is pushed to one side of the wellbore to tilt the drill bit. Orientation equipment typically comprises a standard mule-shoe sub for magnetic orientation. This basic system concept has been around for decades; however problems with angle build and directional control have limited its commercial success.
U.S. Pat. No. 5,213,168 to Warren et. al. (assigned to Amoco Corporation) describes an improved curved drilling assembly. Consistent performance was achieved, in part, by stabilizing the drill bit to continually point along a curved path and designing the bit so that it cuts only in the direction it is pointed. In particular, improved bit stability was achieved by using a "low-friction gauge" technique. (See, for example, U.S. Pat. Nos. 5,010,789 and 5,042,596 to Brett et. al. and assigned to Amoco Corporation). The drill bit cutters are positioned so that they direct a lateral force toward a smooth pad on the side or gauge portion of the drill bit. The pad contacts the borehole wall and transmits a restoring force to the drill bit. This force rotates with the bit and continually pushes one side of the drill bit (i.e., the one that does not have a gauge cutting structure) against the borehole wall. When such a drill bit is used, the curve drilling assembly drills a curved path by continually pointing the drill bit along a line that is tangent to the curved path. The assembly runs smoothly, the hole is uniform in diameter, and the effects of varying lithology are negated. Moreover, the cost to manufacture such an assembly, including the anti-whirl drill bit, is much less than that for a curve drilling assembly that uses a mud motor.
When the drill bit rotates about its center in a gauge-hole, the off-center position of the flexible joint causes the drill bit axis to be tilted with respect to the borehole centerline everywhere except at the bit face. At the bit face, the centerline of the drill bit is pointed along a tangent to the curve centerline. If the curvature of the hole is perturbed and becomes less than the desired curvature, the drill bit axis will point above the borehole inclination and will thus tend to increase the curvature. If the curvature becomes greater than that which is desired, the opposite occurs. Thus, stable equilibrium results when the bit face centerline and hole inclination are aligned. Moreover, as the bit drills ahead along a curved path, the inclination of the bit continually changes so that it is always inclined in a direction that keeps the borehole along the desired curved path without requiring the bit to cut sideways.
Although the drilling system of U.S. Pat. No. 5,213,168 has many advantages over the prior art, experience has shown that there is still room for improvement.